The tissues of the human oral cavity are frequently exposed to repeated low doses of known and suspected carcinogens such as tobacco and x-radiation from diagnostic medical and dental sources. Prior studies in our laboratory have shown synergy between concurrent applications of DMBA and exposures to modest levels of x-radiation in hamster cheek pouch tumorigenesis. This synergy was manifested by greater tumor incidence, shorter latent period, and greater tumor volume in animals receiving combined treatments than in animals receiving either treatment alone. We have also shown that low-level x-radiation causes a significant late increase in pouch vascular volume and that there is a significant interaction between the effects of x-radiation and DMBA-induced tumors on pouch functional vasculature. Ongoing studies are examining the effects of decreasing radiation doses on DMBA-induced tumor incidence and progression as well as the possible mechanistic role of radiation-induced hemodynamic changes in the induction and progression of these tumors. Young adult male Syrian golden hamsters have received various doses of DMBA to the cheek pouch accompanied by repeated exposures of 1-20R x-radiation at weekly intervals. The temporal relationships between applications of chemical and radiation have varied. Animals are being observed for varying lengths of time; tumor latency, volume, incidence, histopathology and hemodynamics are being studied using observational, histopathological and radioisotopic techniques. It is anticipated that the results of these studies will clarify dose-response relationships and time-dose relationships of low level x-radiation in this system, leading to a more quantitative basis for calculations of human radiation risk estimates. Additionally, the data should provide information on the nature of radiation induced hemodynamic changes in the induction of epithelial neoplasms by low level exposures to multiple environmental carcinogens.